Will my portal show my customer how much baggage is included before they pay for a flight?

Yes, in most cases your portal can show the baggage allowance before your customer pays, because the airline sends that information along with the fare.

When a fare comes back from the supplier, it usually carries the cabin and checked baggage that the airline includes with that particular fare, and your portal can display it on the flight result and again on the review page before payment. This matters more than it used to, because airlines now sell the same route as several different fares, where the cheaper one carries less baggage than the one just above it, and your customer needs to see that difference to choose properly.

Being honest about the limits, how complete this is depends on the supplier and on the airline. Some airlines publish a clean baggage allowance that displays neatly on the page, while others send it only inside the written fare rules, which read as a block of text rather than something your portal can put in a tidy line. Low cost carriers often include very little in the base fare and expect baggage to be bought as an extra, and whether your customer can buy that extra bag during booking, rather than later at the airport, depends on whether your supplier supports paid add ons for that airline.

Before you go live, run a few searches on the routes you actually sell, on a full service airline and on a low cost one, and check what your portal shows at the result stage and on the payment page. If the allowance is missing for an airline you sell often, raise it with your supplier, since it is usually a matter of what their fare response carries rather than something your portal is holding back.